One of the other things NOT being mentioned, is the ability of the pin in the receiver. many if not most 5/8" pins are rated to 10K of trailer. At least per a body builder that built the dump unit/hitch for my ihc. That is my limit on that rig. Granted I do pull my 12K trailer.....but he would recommend a 3/4" pin for that sized trailer. There is more than one factor involved into IF a hitch receiver etc will handle a given load.
An old rusty hitch may also be the wrong unit, as the rust may be all that is there vs solid metal also. If the op wants to use the hitch he has, great, if not, great.........
Marty